SOLUTION 8 1. a+ M B = 0; N A = 0. N A = kn = 16.5 kn. Ans. N B = 0.

Transcription

1 8 1. The mine car and its contents have a total mass of 6 Mg and a center of gravity at G. If the coefficient of static friction between the wheels and the tracks is m s = 0.4 when the wheels are locked, find the normal force acting on the front wheels at and the rear wheels at when the brakes at both and are locked. Does the car move? 10 kn 0.9 m G 0.15 m Equations of Equilibrium: The normal reactions acting on the wheels at ( and ) are independent as to whether the wheels are locked or not. Hence, the normal reactions acting on the wheels are the same for both cases. 0.6 m 1.5 m a+ M = 0; N = 0 N = kn = 16.5 kn + c F y = 0; N = 0 N = kn = 42.3 kn When both wheels at and are locked, then 1F 2 max = m s N = = kn and 1F 2 max = m s N = = kn. Since 1F 2 max + F max = kn 7 10 kn, the wheels do not slip. Thus, the mine car does not move.

2 8 2. Determine the maximum force P the connection can support so that no slipping occurs between the plates. There are four bolts used for the connection and each is tightened so that it is subjected to a tension of 4 kn. The coefficient of static friction between the plates is m s = 0.4. P 2 P 2 P Free-ody Diagram: The normal reaction acting on the contacting surface is equal to the sum total tension of the bolts. Thus, N = 4(4) kn = 16 kn. When the plate is on the verge of slipping, the magnitude of the friction force acting on each contact surface can be computed using the friction formula F = m s N = 0.4(16) kn. s indicated on the free-body diagram of the upper plate, F acts to the right since the plate has a tendency to move to the left. Equations of Equilibrium: : + F x = 0; 0.4(16) - P 2 = 0 p = 12.8 kn

3 8 3. The winch on the truck is used to hoist the garbage bin onto the bed of the truck. If the loaded bin has a weight of 8500 lb and center of gravity at G, determine the force in the cable needed to begin the lift. The coefficients of static friction at and are m = 0.3 and m = 0.2, respectively. Neglect the height of the support at. G 10 ft 12 ft 30 a+ M = 0; 8500(12) - N (22) = 0 N = lb : + F x = 0; T cos N cos 30 - N sin ( ) = 0 T( ) N = c F y = 0; T sin 30 + N cos N sin 30 = 0 T(0.5) N = Solving: T = lb = 3.67 kip N = lb

4 *8 4. The tractor has a weight of 4500 lb with center of gravity at G. The driving traction is developed at the rear wheels, while the front wheels at are free to roll. If the coefficient of static friction between the wheels at and the ground is m s = 0.5, determine if it is possible to pull at P = 1200 lb without causing the wheels at to slip or the front wheels at to lift off the ground. G 3.5 ft 1.25 ft P Slipping: 4ft 2.5 ft a+ M = 0; P N = 0 : + F x = 0; P = 0.5 N P = lb N = lb Tipping 1N = 02 a+ M = 0; -P = 0 P = 9000 lb Since P Req d = 1200 lb lb It is possible to pull the load without slipping or tipping.

5 8 5. The 15-ft ladder has a uniform weight of 80 lb and rests against the smooth wall at. If the coefficient of static friction at is m = 0.4, determine if the ladder will slip. Take u = ft a + M = 0; : + F x = 0; N 115 sin cos 60 = 0 N = lb F = lb θ + c F y = 0; N = 80 lb 1F 2 max = = 32 lb lb (O.K!) The ladder will not slip.

6 8 6. The ladder has a uniform weight of 80 lb and rests against the wall at. If the coefficient of static friction at and is m = 0.4, determine the smallest angle u at which the ladder will not slip. 15 ft Free-ody Diagram: Since the ladder is required to be on the verge to slide down, the frictional force at and must act to the right and upward respectively and their magnitude can be computed using friction formula as indicated on the FD, Fig. a. u (Ff) = mn = 0.4 N (Ff) = mn = 0.4 N Equations of Equlibrium: Referring to Fig. a. 0.4N - N = 0 + c Fy = 0; N + 0.4N - 80 = 0 N = 0.4 N (1) T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) + Fx = 0; : (2) Solving Eqs. (1) and (2) yields N = lb Using these results, a + M = 0; N = lb 0.4(27.59)(15 cos u) (15 sin u) - 80 cos u(7.5) = sin u cos u = sin u = = 1.05 tan u = cos u u = 46.4

7 8 7. The block brake consists of a pin-connected lever and friction block at. The coefficient of static friction between the wheel and the lever is ms = 0.3, and a torque of 5N# m is applied to the wheel. Determine if the brake can hold the wheel stationary when the force applied to the lever is (a) P = 30 N, (b) P = 70 N. 50 mm 150 mm O 5N m P 200 mm 400 mm To hold lever: a+ M O = 0; F (0.15) - 5 = 0; F = N Require Lever, a+ M = 0; P Reqd. (0.6) (0.2) (0.05) = 0 P Reqd. = 39.8 N N = N 0.3 = N a) P = 30 N N No b) P = 70 N N Yes

8 *8 8. The block brake consists of a pin-connected lever and friction block at. The coefficient of static friction between the wheel and the lever is ms = 0.3, and a torque of 5N# m is applied to the wheel. Determine if the brake can hold the wheel stationary when the force applied to the lever is (a) P = 30 N, (b) P = 70 N. 50 mm 5N m 150 mm O 200 mm 400 mm P To hold lever: a+ M O = 0; -F (0.15) + 5 = 0; F = N Require N = N 0.3 Lever, a+ M = 0; P Reqd. (0.6) (0.2) (0.05) = 0 P Reqd. = N = N a) P = 30 N N No b) P = 70 N N Yes

9 8 9. The block brake is used to stop the wheel from rotating when the wheel is subjected to a couple moment M 0. If the coefficient of static friction between the wheel and the block is m s, determine the smallest force P that should be applied. P a b C c O M 0 a+ M C = 0; Pa - Nb + m s Nc = 0 r N = Pa (b - m s c) a+ M O = 0; m s Nr - M 0 = 0 a m s P b - m s c r = M 0 P = M 0 m s ra (b - m s c)

10 8 10. The block brake is used to stop the wheel from rotating when the wheel is subjected to a couple moment M 0. If the coefficient of static friction between the wheel and the block is m s, show that the brake is self locking, i.e., the required force P 0, provided b>c m s. P a b C c O M 0 Require P 0. Then, from Soln. 8 9 r b m s c m s Ú b c

11 8 11. The block brake is used to stop the wheel from rotating when the wheel is subjected to a couple moment M 0. If the coefficient of static friction between the wheel and the block is m s, determine the smallest force P that should be applied. P a b C c a+ M C = 0; Pa - Nb - m s Nc = 0 N = Pa (b + m s c) c+ M O = 0; m s Nr - M 0 = 0 a m s P a b + m s c br = M 0 P = M 0 m s ra (b + m s c) M 0 r O

12 *8 12. If a torque of M = 300 N # m is applied to the flywheel, determine the force that must be developed in the hydraulic cylinder CD to prevent the flywheel from rotating. The coefficient of static friction between the friction pad at and the flywheel is ms = 0.4. D 0.6 m 30 1m C Free-odyDiagram: First we will consider the equilibrium of the flywheel using the free-body diagram shown in Fig. a. Here, the frictional force F must act to the left to produce the counterclockwise moment opposing the impending clockwise rotational motion caused by the 300 N # m couple moment. Since the wheel is required to be on the verge of slipping, then F = msn = 0.4 N. Subsequently, the free-body diagram of member C shown in Fig. b will be used to determine FCD. 60 mm M 300 N m 0.3 m O Equations of Equilibrium: We have Using this result, a + M = 0; 0.4 N(0.3) = 0 N = 2500 N T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) a + MO = 0; FCD sin 30 (1.6) + 0.4(2500)(0.06) (1) = 0 FCD = 3050 N = 3.05 kn

13 8 13. The cam is subjected to a couple moment of 5N# m. Determine the minimum force P that should be applied to the follower in order to hold the cam in the position shown. The coefficient of static friction between the cam and the follower is m s = 0.4. The guide at is smooth. P 10 mm 60 mm O 5N m Cam: a+ M O = 0; N (0.06) (N ) = 0 N = N Follower: + c F y = 0; P = 0 P = 147 N

14 8 14. Determine the maximum weight W the man can lift with constant velocity using the pulley system, without and then with the leading block or pulley at. The man has a weight of 200 lb and the coefficient of static friction between his feet and the ground is m s = 0.6. C w 45 C w (a) (b) a) + c F y = 0; W 3 sin 45 + N = 0 :+ F x = 0; - W 3 cos N = 0 W = 318 lb b) + c F y = 0; : + F x = 0; N = 200 lb = W 3 W = 360 lb

15 8 15. The car has a mass of 1.6 Mg and center of mass at G. If the coefficient of static friction between the shoulder of the road and the tires is m s = 0.4, determine the greatest slope u the shoulder can have without causing the car to slip or tip over if the car travels along the shoulder at constant velocity. G 2.5 ft 5ft Tipping: θ a+ M = 0; -W cos u W sin u12.52 = 0 tan u = 1 u = 45 Slipping: Q+ F x = 0; a+ F y = 0; 0.4 N - W sin u = 0 N - W cos u = 0 tan u = 0.4 u = 21.8 (car slips before it tips)

16 *8 16. The uniform dresser has a weight of 90 lb and rests on a tile floor for which m s = If the man pushes on it in the horizontal direction u = 0, determine the smallest magnitude of force F needed to move the dresser. lso, if the man has a weight of 150 lb, determine the smallest coefficient of static friction between his shoes and the floor so that he does not slip. Dresser: + c F y = 0; N D - 90 = 0 N D = 90 lb : + F x = 0; F (90) = 0 u F F = 22.5 lb Man: + c F y = 0; N m = 0 N m = 150 lb : + F x = 0; m m (150) = 0 m m = 0.15

17 8 17. The uniform dresser has a weight of 90 lb and rests on a tile floor for which m s = If the man pushes on it in the direction u = 30, determine the smallest magnitude of force F needed to move the dresser. lso, if the man has a weight of 150 lb, determine the smallest coefficient of static friction between his shoes and the floor so that he does not slip. u F Dresser: + c F y = 0; N F sin 30 = 0 : + F x = 0; F cos N = 0 N = lb F = lb = 30.4 lb Man: + c F y = 0; N m sin 30 = 0 : + F x = 0; F m cos 30 = 0 N m = lb F m = lb m m = F m N m = = 0.195

18 8 18. The 5-kg cylinder is suspended from two equal-length cords. The end of each cord is attached to a ring of negligible mass that passes along a horizontal shaft. If the rings can be separated by the greatest distance d = 400 mm and still support the cylinder, determine the coefficient of static friction between each ring and the shaft. d 600 mm 600 mm Equilibrium of the Cylinder: Referring to the FD shown in Fig. a, + c F y = 0; 2T 232 R - m(9.81) = 0 6 T = m Equilibrium of the Ring: Since the ring is required to be on the verge to slide, the frictional force can be computed using friction formula F f = mn as indicated in the FD of the ring shown in Fig. b.using the result of I, + c F y = 0; : + F x = 0; N m = 0 m(4.905 m) m 2 6 = 0 N = m m = 0.354

19 8 19. The 5-kg cylinder is suspended from two equal-length cords. The end of each cord is attached to a ring of negligible mass, which passes along a horizontal shaft. If the coefficient of static friction between each ring and the shaft is m s = 0.5, determine the greatest distance d by which the rings can be separated and still support the cylinder. d Friction: When the ring is on the verge to sliding along the rod, slipping will have to occur. Hence, F = mn = 0.5N. From the force diagram (T is the tension developed by the cord) Geometry: tan u = N 0.5N = 2 u = mm 600 mm d = cos = 537 mm

20 *8 20. The board can be adjusted vertically by tilting it up and sliding the smooth pin along the vertical guide G. When placed horizontally, the bottom C then bears along the edge of the guide, where m s = 0.4. Determine the largest dimension d which will support any applied force F without causing the board to slip downward. + c F y = 0; 0.4N C - F = 0 G Top view 0.75 in. C 0.75 in. G 6in. Side view F d a+ M = 0; -F(6) + d(n C ) - 0.4N C (0.75) = 0 Thus, -0.4N C (6) + d(n C ) - 0.4N C (0.75) = 0 d = 2.70 in. -

21 8 21. The uniform pole has a weight W and length L. Its end is tied to a supporting cord, and end is placed against the wall, for which the coefficient of static friction is m s. Determine the largest angle u at which the pole can be placed without slipping. C L a + M = 0; -N (L cos u) - m s N (L sin u) + W a L (1) 2 sin ub = 0 : + F x = 0; N - T sin u 2 = 0 (2) u L + c F y = 0; m s N - W + T cos u 2 = 0 (3) Substitute Eq. (2) into Eq. (3): m s T sin u 2 - W + T cos u 2 = 0 W = Tacos u 2 + m s sin u 2 b Substitute Eqs. (2) and (3) into Eq. (1): T sin u 2 cos u - T cos u 2 sin u + W 2 sin u = 0 Substitute Eq. (4) into Eq. (5): sin u 2 cos u - cos u 2 sin u cos u 2 sin u m s sin u 2 sin u = 0 -sin u acos u 2 + m s sin u 2 bsin u = 0 cos u 2 + m s sin u 2 = 1 cos 2 u 2 + m s sin u 2 cos u 2 = 1 m s sin u 2 cos u 2 = sin2 u 2 cos u 2 (4) (5) tan u 2 = m s u = 2 tan - 1 m s lso, because we have a three force member, L 2 = L 2 cos u + tan fa L sin ub 2 1 = cos u + m s sin u m s = 1 - cos u sin u = tan u 2 u = 2 tan -1 m s

22 8 22. If the clamping force is F = 200 N and each board has a mass of 2 kg, determine the maximum number of boards the clamp can support. The coefficient of static friction between the boards is ms = 0.3, and the coefficient of static friction between the boards and the clamp is ms = F F Free-ody Diagram: The boards could be on the verge of slipping between the two boards at the ends or between the clamp. Let n be the number of boards between the clamp. Thus, the number of boards between the two boards at the ends is n - 2. If the boards slip between the two end boards, then F = msn = 0.3(200) = 60 N. Equations of Equilibrium: Referring to the free-body diagram shown in Fig. a, we have n = (60) - (n - 2)(2)(9.81) = 0 T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) + c Fy = 0; If the end boards slip at the clamp, then F = ms N = 0.45(200) = 90 N. y referring to the free-body diagram shown in Fig. b, we have a + c Fy = 0; 2(90) - n(2)(9.81) = 0 n = 9.17 Thus, the maximum number of boards that can be supported by the clamp will be the smallest value of n obtained above, which gives n = 8 a + Mclamp = 0; 60 - (2)(9.81)(n - 1)2 = (n - 1) = 0 n = 7.12 n = 7

23 kg disk rests on an inclined surface for which m s = 0.2. Determine the maximum vertical force P that may be applied to link without causing the disk to slip at C. P 200 mm 300 mm 600 mm 200 mm Equations of Equilibrium: From FD (a), C 30 a+ M = 0; P y = 0 y = P From FD (b), + c F y = 0 N C sin 60 - F C sin P = 0 (1) a + M O = 0; F C P12002 = 0 (2) Friction: If the disk is on the verge of moving, slipping would have to occur at point C. Hence, F C = m s N C = 0.2N C. Substituting this value into Eqs. (1) and (2) and solving, we have P = 182 N N C = N

24 *8 24. The man has a weight of 200 lb, and the coefficient of static friction between his shoes and the floor is m s = 0.5. Determine where he should position his center of gravity G at d in order to exert the maximum horizontal force on the door. What is this force? G F max = 0.5 N = 0.5(200) = 100 lb 3ft : + F x = 0; P = 0; P = 100 lb a+ M O = 0; 200(d) - 100(3) = 0 d = 1.50 ft d

25 8 25. The crate has a weight of W = 150 lb, and the coefficients of static and kinetic friction are m s = 0.3 and m k = 0.2, respectively. Determine the friction force on the floor if u = 30 and P = 200 lb. u P Equations of Equilibrium: Referring to the FD of the crate shown in Fig. a, + c F y = 0; N sin = 0 N = 50 lb : + F x = 0; 200 cos 30 - F = 0 F = lb Friction Formula: Here, the maximum frictional force that can be developed is (F f ) max = m s N = 0.3(50) = 15 lb Since F = lb 7 (F f ) max, the crate will slide. Thus the frictional force developed is F f = m k N = 0.2(50) = 10 lb

26 8 26. The crate has a weight of W = 350 lb, and the coefficients of static and kinetic friction are m s = 0.3 and m k = 0.2, respectively. Determine the friction force on the floor if u = 45 and P = 100 lb. u P Equations of Equilibrium: Referring to the FD of the crate shown in Fig. a, + c F y = 0; N sin = 0 N = lb : + F x = 0; 100 cos 45 - F = 0 F = lb Friction Formula: Here, the maximum frictional force that can be developed is (F f ) max = m s N = 0.3(279.29) = lb Since F = lb 6 (F f ) max, the crate will not slide. Thus, the frictional force developed is F f = F = 70.7 lb

27 8 27. The crate has a weight W and the coefficient of static friction at the surface is m s = 0.3. Determine the orientation of the cord and the smallest possible force P that has to be applied to the cord so that the crate is on the verge of moving. θ P Equations of Equilibrium: + c F y = 0; : + F x = 0; N + P sin u - W = 0 P cos u - F = 0 (1) (2) Friction: If the crate is on the verge of moving, slipping will have to occur. Hence, F = m s N = 0.3N. Substituting this value into Eqs. (1)and (2) and solving, we have dp In order to obtain the minimum P, du = 0. d 2 P du 2 P = 0.3W cos u sin u dp du = 0.3W sin u cos u 1cos u sin u2 R 2 = 0 1cos u sin u sin u cos u2 2 = 0.3W R 1cos u sin u2 3 d 2 t u = 16.70, P = W 7 0. Thus, u = will result in a minimum P. 2 du P = N = sin u cos u = 0 u = = 16.7 W cos u cos u sin u 0.3W cos sin = 0.287W

28 *8 28. If the coefficient of static friction between the man s shoes and the pole is m s = 0.6, determine the minimum coefficient of static friction required between the belt and the pole at in order to support the man. The man has a weight of 180 lb and a center of gravity at G. 4 ft G Free-ody Diagram: The man s shoe and the belt have a tendency to slip downward. Thus, the frictional forces F and F C must act upward as indicated on the free-body diagram of the man shown in Fig. a. Here, F C is required to develop to its maximum, thus F C = (m s ) C N C = 0.6N C. C Equations of Equilibrium: Referring to Fig. a, we have a+ M = 0; N C (4) + 0.6N C (0.75) - 180(3.25) = 0 2 ft 0.75 ft 0.5 ft N C = lb : + F x = 0; + c F y = 0; N = 0 F + 0.6(131.46) = 0 To prevent the belt from slipping the coefficient of static friction at contact point must be at least (m s ) = F N = = N = lb F = lb

29 8 29. The friction pawl is pinned at and rests against the wheel at. It allows freedom of movement when the wheel is rotating counterclockwise about C. Clockwise rotation is prevented due to friction of the pawl which tends to bind the wheel. If 1m s 2 = 0.6, determine the design angle u which will prevent clockwise motion for any value of applied moment M. Hint: Neglect the weight of the pawl so that it becomes a two-force member. Friction: When the wheel is on the verge of rotating, slipping would have to occur. Hence, F = mn = 0.6N. From the force diagram ( F is the force developed in the two force member ) M θ 20 C tan120 + u2 = 0.6N N = 0.6 u = 11.0

30 8 30. If u = 30 determine the minimum coefficient of static friction at and so that equilibrium of the supporting frame is maintained regardless of the mass of the cylinder C. Neglect the mass of the rods. C u L u L Free-ody Diagram: Due to the symmetrical loading and system, ends and of the rod will slip simultaneously. Since end tends to move to the right, the friction force F must act to the left as indicated on the free-body diagram shown in Fig. a. Equations of Equilibrium: We have + Fx = 0; : FC sin 30 - F = 0 F = 0.5FC + c Fy = 0; N - FC cos 30 = 0 N = FC ms = T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) Therefore, to prevent slipping the coefficient of static friction ends and must be at least F 0.5FC = = N FC

31 8 31. If the coefficient of static friction at and is ms = 0.6, determine the maximum angle u so that the frame remains in equilibrium, regardless of the mass of the cylinder. Neglect the mass of the rods. C u L u L Free-ody Diagram: Due to the symmetrical loading and system, ends and of the rod will slip simultaneously. Since end is on the verge of sliding to the right, the friction force F must act to the left such that F = msn = 0.6N as indicated on the free-body diagram shown in Fig. a. Equations of Equilibrium: We have + c Fy = 0; N - FC cos u = 0 + Fx = 0; : FC sin u - 0.6(FC cos u) = 0 N = FC cos u T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) tan u = 0.6 u = 31.0

32 *8 32. The semicylinder of mass m and radius r lies on the rough inclined plane for which f = 10 and the coefficient of static friction is m s = 0.3. Determine if the semicylinder slides down the plane, and if not, find the angle of tip u of its base. θ r Equations of Equilibrium: φ a + M F1r2-9.81m sin ua 4r O = 0; (1) 3p b = 0 : + F x = 0; F cos 10 - N sin 10 = 0 (2) + c F y = 0 F sin 10 + N cos m = 0 (3) Solving Eqs. (1), (2) and (3) yields N = 9.661m F = 1.703m u = 24.2 Friction: The maximum friction force that can be developed between the semicylinder and the inclined plane is F max = mn = m = 2.898m. Since F max 7 F = 1.703m, the semicylinder will not slide down the plane.

33 8 33. The semicylinder of mass m and radius r lies on the rough inclined plane. If the inclination f = 15, determine the smallest coefficient of static friction which will prevent the semicylinder from slipping. θ r Equations of Equilibrium: φ +Q F x = 0; a+ F y = 0; F m sin 15 = 0 F = 2.539m N m cos 15 = 0 N = 9.476m Friction: If the semicylinder is on the verge of moving, slipping would have to occur. Hence, F = m s N 2.539m = m s m2 m s = 0.268

34 8 34. The coefficient of static friction between the 150-kg crate and the ground is m s = 0.3, while the coefficient of static friction between the 80-kg man s shoes and the ground is œ m s = 0.4. Determine if the man can move the crate. Free - ody Diagram: Since P tends to move the crate to the right, the frictional force F C will act to the left as indicated on the free - body diagram shown in Fig. a. Since the crate is required to be on the verge of sliding the magnitude of F C can be computed using the friction formula, i.e. F C = m s N C = 0.3 N C. s indicated on the free - body diagram of the man shown in Fig. b, the frictional force F m acts to the right since force P has the tendency to cause the man to slip to the left. 30 Equations of Equilibrium: Referring to Fig. a, + c F y = 0; N C + P sin (9.81) = 0 : + F x = 0; P cos N C = 0 Solving, P = N N C = N Using the result of P and referring to Fig. b, we have + c F y = 0; N m sin 30-80(9.81) = 0 N m = N : + F x = 0; F m cos 30 = 0 F m = N Since F m 6 F max = m s N m = 0.4( ) = N, the man does not slip. Thus, he can move the crate.

35 8 35. If the coefficient of static friction between the crate and the ground is m s = 0.3, determine the minimum coefficient of static friction between the man s shoes and the ground so that the man can move the crate. Free - ody Diagram: Since force P tends to move the crate to the right, the frictional force F C will act to the left as indicated on the free - body diagram shown in Fig. a. Since the crate is required to be on the verge of sliding, F C = m s N C = 0.3 N C. s indicated on the free - body diagram of the man shown in Fig. b, the frictional force F m acts to the right since force P has the tendency to cause the man to slip to the left. 30 Equations of Equilibrium: Referring to Fig. a, + c F y = 0; N C + P sin (9.81) = 0 : + F x = 0; P cos N C = 0 Solving yields P = N N C = N Using the result of P and referring to Fig. b, + c F y = 0; N m sin 30-80(9.81) = 0 N m = N : + F x = 0; F m cos 30 = 0 F m = N Thus, the required minimum coefficient of static friction between the man s shoes and the ground is given by m s = F m N m = = 0.376

36 *8 36. The thin rod has a weight W and rests against the floor and wall for which the coefficients of static friction are m and m, respectively. Determine the smallest value of u for which the rod will not move. L Equations of Equilibrium: u : + F x = 0; F - N = 0 + c F y = 0 N + F - W = 0 (1) (2) a + M = 0; N (L sin u) + F (cos u)l - W cos ua L (3) 2 b = 0 Friction: If the rod is on the verge of moving, slipping will have to occur at points and. Hence, F = m N and F = m N. Substituting these values into Eqs. (1), (2), and (3) and solving we have N = W 1 + m m N = m W 1 + m m u = tan - 1 a 1 - m m b 2m

37 8 37. The 80-lb boy stands on the beam and pulls on the cord with a force large enough to just cause him to slip. If the coefficient of static friction between his shoes and the beam is (m s ) D = 0.4, determine the reactions at and. The beam is uniform and has a weight of 100 lb. Neglect the size of the pulleys and the thickness of the beam D C Equations of Equilibrium and Friction: When the boy is on the verge of slipping, then F D = (m s ) D N D = 0.4N D. From FD (a), 5ft 1ft 3ft 4ft + c F y = 0; N D - Ta 5 13 b - 80 = 0 (1) : + F x = 0; 0.4N D - Ta b = 0 (2) Solving Eqs. (1) and (2) yields Hence, F D = 0.4(96.0) = 38.4 lb. From FD (b), a+ M = 0; T = 41.6 lb N D = 96.0 lb 100(6.5) (8) a 5 13 b(13) (13) sin 30 (7) - y (4) = 0 y = lb = 474 lb : + F x = 0; x a 12 b cos 30 = 0 13 x = 36.0 lb y = lb = 232 lb + c F y = 0; a 5 13 b sin y = 0

38 8 38. The 80-lb boy stands on the beam and pulls with a force of 40 lb. If (m s ) D = 0.4, determine the frictional force between his shoes and the beam and the reactions at and.the beam is uniform and has a weight of 100 lb. Neglect the size of the pulleys and the thickness of the beam D C 60 Equations of Equilibrium and Friction: From FD (a), + c F y = 0; N D - 40a 5 13 b - 80 = 0 N D = lb 5ft 1ft 3ft 4ft : + F x = 0; F D - 40a b = 0 F D = lb Since (F D ) max = (m s )N D = 0.4(95.38) = lb 7 F D, then the boy does not slip. Therefore, the friction force developed is From FD (b), a+ M = 0; F D = lb = 36.9 lb 100(6.5) (8) - 40a 5 13 b(13) + 40(13) + 40 sin 30 (7) - y (4) = 0 y = lb = 468 lb : + F x = 0; x + 40a 12 b cos 30 = 0 13 x = lb = 34.6 lb + c F y = 0; a 5 13 b sin y = 0 y = lb = 228 lb

39 8 39. Determine the smallest force the man must exert on the rope in order to move the 80-kg crate. lso, what is the angle u at this moment? The coefficient of static friction between the crate and the floor is m s = u Crate: : + F x = 0; 0.3N C - T sin u = 0 + c F y = 0; N C + T cos u - 80(9.81) = 0 (1) (2) Pulley: : + F x = 0; -T cos 30 + T cos 45 + T sin u = 0 + c F y = 0; T sin 30 + T sin 45 - T cos u = 0 Thus, T = T sin u T = T cos u u = tan - 1 a b = 7.50 T = T From Eqs. (1) and (2), N C = 239 N T =550 N So that T = 452 N (3)

40 *8 40. Two blocks and have a weight of 10 lb and 6 lb, respectively. They are resting on the incline for which the coefficients of static friction are m = 0.15 and m = Determine the incline angle u for which both blocks begin to slide. lso find the required stretch or compression in the connecting spring for this to occur. The spring has a stiffness of k = 2lb>ft. k u 2lb/ft Equations of Equilibrium: Using the spring force formula, FD (a), +Q F x = 0; 2x + F - 10 sin u = 0 a+ F y = 0; N - 10 cos u = 0 F sp = kx = 2x, from (1) (2) From FD (b), +Q F x = 0; F - 2x - 6 sin u = 0 (3) a+ F y = 0; N - 6 cos u = 0 (4) Friction: If block and are on the verge to move, slipping would have to occur at point and. Hence. F = m s N = 0.15N and F = m s N = 0.25N. Substituting these values into Eqs. (1), (2),(3) and (4) and solving, we have u = 10.6 x = ft N = lb N = lb

41 8 41. Two blocks and have a weight of 10 lb and 6 lb, respectively. They are resting on the incline for which the coefficients of static friction are m = 0.15 and m = Determine the angle u which will cause motion of one of the blocks. What is the friction force under each of the blocks when this occurs? The spring has a stiffness of k = 2lb>ft and is originally unstretched. k u 2lb/ft Equations of Equilibrium: Since neither block nor block is moving yet, the spring force F sp = 0. From FD (a), +Q F x = 0; F - 10 sin u = 0 a+ F y = 0; N - 10 cos u = 0 (1) (2) From FD (b), +Q F x = 0; F - 6 sin u = 0 a+ F y = 0; N - 6 cos u = 0 Friction: ssuming block is on the verge of slipping, then F = m N = 0.15N Solving Eqs. (1),(2),(3),(4), and (5) yields u = N = lb F = lb F = lb N = lb Since (F ) max = m N = 0.25(5.934) = lb 7 F, block does not slip. Therefore, the above assumption is correct. Thus u = 8.53 F = 1.48 lb F = lb (3) (4) (5)

42 8 42. The friction hook is made from a fixed frame which is shown colored and a cylinder of negligible weight. piece of paper is placed between the smooth wall and the cylinder. If u = 20, determine the smallest coefficient of static friction m at all points of contact so that any weight W of paper p can be held. Paper: + c F y = 0; F = 0.5W u p W F = mn; F = mn Cylinder: N = 0.5W m a+ M O = 0; F = 0.5W : + F x = 0; N cos 20 + F sin W m = 0 + c F y = 0; N sin 20 - F cos W = 0 F = mn; m 2 sin m cos 20 - sin 20 = 0 m = 0.176

43 8 43. The uniform rod has a mass of 10 kg and rests on the inside of the smooth ring at and on the ground at. If the rod is on the verge of slipping, determine the coefficient of static friction between the rod and the ground. 0.5 m C a+ m = 0; + c F y = 0; : + F x = 0; N (0.4) (0.25 cos 30 ) = 0 N = N N cos 30 = 0 N = N m(52.12) sin 30 = 0 m = m

44 *8 44. The rings and C each weigh W and rest on the rod, which has a coefficient of static friction of ms. If the suspended ring at has a weight of 2W, determine the largest distance d between and C so that no motion occurs. Neglect the weight of the wire. The wire is smooth and has a total length of l. d C Free-ody Diagram: The tension developed in the wire can be obtained by considering the equilibrium of the free-body diagram shown in Fig. a. + c Fy = 0; 2T sin u - 2w = 0 T = w sin u T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) Due to the symmetrical loading and system, rings and C will slip simultaneously. Thus, it s sufficient to consider the equilibrium of either ring. Here, the equilibrium of ring C will be considered. Since ring C is required to be on the verge of sliding to the left, the friction force FC must act to the right such that FC = msnc as indicated on the free-body diagram of the ring shown in Fig. b. Equations of Equilibrium: Using the result of T and referring to Fig. b, we have + c Fy = 0; + Fx = 0; : NC - w - c ms(2w) - c tan u = W d sin u = 0 sin u NC = 2w W d cos u = 0 sin u 1 2ms d 2 l 2 a b - a b 2 2l2 - d2 2. = From the geometry of Fig. c, we find that tan u = d d 2 Thus, 2l2 - d2 1 = d 2ms d = 2msl ms 2

45 8 45. The three bars have a weight of W = 20 lb, W = 40 lb, and W C = 60 lb, respectively. If the coefficients of static friction at the surfaces of contact are as shown, determine the smallest horizontal force P needed to move block C μ C = 0.5 μ = 0.3 μ D = 0.2 D P Equations of Equilibrium and Friction: If blocks and move together, then slipping will have to occur at the contact surfaces C and D. Hence, F C = m sc N C = 0.5N C a nd F D = m sd N D = 0.2N D. From FD (a) + c F y = 0; :+ F x = 0; N C - Ta 8 17 b - 60 = 0 0.5N C - Ta b = 0 (1) (2) and FD (b) + c F y = 0; : + F x = 0; Solving Eqs. (1), (2), (3), and (4) yields If only block moves, then slipping will have to occur at contact surfaces and D. Hence, F = m s N = 0.3N and F D = m sd N D = 0.2N D. From FD (c) + c F y = 0; :+ F x = 0; and FD (d) + c F y = 0; : + F x = 0; N D - N C - 60 = 0 P - 0.5N C - 0.2N D = 0 T = lb N C = lb N D = lb P = lb N - Ta 8 17 b = 0 0.3N - Ta b = 0 N D - N - 20 = 0 P - 0.3N - 0.2N D = 0 Solving Eqs. (5),(6),(7), and (8) yields T = lb N = lb N D = lb (3) (4) (5) (6) (7) (8) P = lb = 63.5 lb (Control!)

46 8 46. The beam has a negligible mass and thickness and is subjected to a triangular distributed loading. It is supported at one end by a pin and at the other end by a post having a mass of 50 kg and negligible thickness. Determine the minimum force P needed to move the post.the coefficients of static friction at and C are m = 0.4 and m C = 0.2, respectively. 2m 400 mm 300 mm 800 N/m P C Member : a+ M = 0; -800a 4 3 b + N (2) = 0 N = N Post: ssume slipping occurs at C; F C = 0.2 N C a+ M C = 0; P(0.3) + F (0.7) = 0 : + F x = 0; + c F y = 0; 4 5 P - F - 0.2N C = P + N C (9.81) = 0 P = 355 N N C = N F = N (F ) max = 0.4(533.3) = N N (O.K.!)

47 8 47. The beam has a negligible mass and thickness and is subjected to a triangular distributed loading. It is supported at one end by a pin and at the other end by a post having a mass of 50 kg and negligible thickness. Determine the two coefficients of static friction at and at C so that when the magnitude of the applied force is increased to P = 150 N, the post slips at both and C simultaneously. 2m 400 mm 300 mm 800 N/m P C Member : a+ M = 0; -800a 4 3 b + N (2) = 0 N = N Post: + c F y = 0; N C a 3 5 b - 50(9.81) = 0 a+ M C = 0; (150)(0.3) + F (0.7) = 0 : + F x = 0; m C = F C N C = = m = F N = = N C = N F = N 4 5 (150) - F C = 0 F C = N

48 *8 48. The beam has a negligible mass and thickness and is subjected to a force of 200 N. It is supported at one end by a pin and at the other end by a spool having a mass of 40 kg. If a cable is wrapped around the inner core of the spool, determine the minimum cable force P needed to move the spool. The coefficients of static friction at and D are m = 0.4 and m D = 0.2, respectively. 200 N 2m 1m 1m 0.1 m 0.3 m D P Equations of Equilibrium: From FD (a), a+ M = 0; From FD (b), N = 0 N = N + c F y = 0 N D = 0 N D = N : + F x = 0; P - F - F D = 0 (1) a + M D = 0; F P10.22 = 0 (2) Friction: ssuming the spool slips at point, then F = ms N = = N. Substituting this value into Eqs. (1) and (2) and solving, we have F D = N P = N = 107 N Since F D max = m sd N D = = N 7 F D, the spool does not slip at point D. Therefore the above assumption is correct.

49 8 49. If each box weighs 150 lb, determine the least horizontal force P that the man must exert on the top box in order to cause motion. The coefficient of static friction between the boxes is m s = 0.5, and the coefficient of static friction œ between the box and the floor is m s = 0.2. P 3ft 4.5 ft Free - ody Diagram: There are three possible motions, namely (1) the top box slides, (2) both boxes slide together as a single unit on the ground, and (3) both boxes tip as a single unit about point. We will assume that both boxes slide together as a single unit such that F = m œ s N = 0.2N as indicated on the free - body diagram shown in Fig. a. 5ft 4.5 ft Equations of Equilibrium: + c F y = 0; N = 0 : + F x = 0; P - 0.2N = 0 a+ M O = 0; 150(x) + 150(x) - P(5) = 0 Solving, N = 300 x = 1ft P = 60 lb Since x ft, both boxes will not tip about point. Using the result of P and considering the equilibrium of the free - body diagram shown in Fig. b, we have + c F y = 0; N -150 = 0 N =150 lb : + F x = 0; 60 - F =0 F =60 lb Since F 6F max = m s N =0.5(150) = 75 lb, the top box will not slide. Thus, the above assumption is correct.

50 8 50. If each box weighs 150 lb, determine the least horizontal force P that the man must exert on the top box in order to cause motion. The coefficient of static friction between the boxes is ms = 0.65, and the coefficient of static friction between the box and the floor is msœ = ft 4.5 ft P 5 ft Free - ody Diagram: There are three possible motions, namely (1) the top box slides, (2) both boxes slide together as a single unit on the ground, and (3) both boxes tip as a single unit about point. We will assume that both boxes tip as a single unit about point. Thus, x = 1.5 ft. 4.5 ft Equations of Equilibrium: Referring to Fig. a, N = 0 + F = 0; : x P - F = 0 a + M = 0; 150(1.5) + 150(1.5) - P(5) = 0 Solving, T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) + c Fy = 0; P = 90 lb N = 300 lb F = 90 lb Since F 6 Fmax = msn = 0.35(300) = 105 lb, both boxes will not slide as a single unit on the floor. Using the result of P and considering the equilibrium of the free body diagram shown in Fig. b, + c Fy = 0; N = 0 N = 150 lb + F = 0; : x 90 - F = 0 F = 90 lb Since F 6 Fmax = msœ N = 0.65(150) = 97.5 lb, the top box will not slide. Thus, the above assumption is correct.

51 8 51. The block of weight W is being pulled up the inclined plane of slope a using a force P. If P acts at the angle f as shown, show that for slipping to occur, P = W sin1a + u2>cos1f - u2, where u is the angle of friction; u = tan -1 m. f P a Q + F x = 0; P cos f - W sin a - mn = 0 + a F y = 0; N - W cos a + P sin f = 0 P cos f - W sin a - m(w cos a - P sin f) = 0 sin a + m cos a P = W a cos f + m sin f b Let m = tan u sin (a + u) P = W a cos (f - u) b (QED)

52 *8 52. Determine the angle f at which P should act on the block so that the magnitude of P is as small as possible to begin pushing the block up the incline. What is the corresponding value of P? The block weighs W and the slope a is known. f P a Slipping occurs u = tan - 1 u. when sin (a + u) P = W a cos (f - u) b where u is the angle of friction dp df sin (a + u) sin (f - u) = 0 sin (a + u) = 0 or sin (f - u) = 0 a = -u sin (a + u) sin (f - u) = W a cos 2 b = 0 (f - u) P = W sin (a + f) f = u

53 8 53. The wheel weighs 20 lb and rests on a surface for which m = 0.2. cord wrapped around it is attached to the top of the 30-lb homogeneous block. If the coefficient of static friction at D is m D = 0.3, determine the smallest vertical force that can be applied tangentially to the wheel which will cause motion to impend. P 1.5 ft 1.5 ft C 3ft D Cylinder : ssume slipping at, F = 0.2 N a+ M = 0; F + T = P : + F x = 0; F = T + c F y = 0; N = 20 + P N = (0.2N ) N = lb F = 6.67 lb T = 6.67 lb P = 13.3 lb : + F x = 0; F D = 6.67 lb + c F y = 0; N D = 30 lb (F D ) max = 0.3(30) = 9lb lb No slipping occurs. a+ M D = 0; -30(x) (3) = 0 x = ft No tipping occurs. = 0.75 ft (O.K.!) (O.K.!)

54 8 54. The uniform beam has a weight W and length 4a. It rests on the fixed rails at and. If the coefficient of static friction at the rails is m s, determine the horizontal force P, applied perpendicular to the face of the beam, which will cause the beam to move. a 3a From FD (a), P + c F = 0; a+ M = 0; N + N - W = 0 -N 13a2 + W12a2 = 0 N = 2 3 W N = 1 3 W Support can sustain twice as much static frictional force as support. From FD (b), + c F = 0; P + F - F = 0 a+ M = 0; -P14a2 + F 13a2 = 0 F = 4 3 P F = 1 3 P The frictional load at is 4 times as great as at. The beam will slip at first. P = 3 4 F max = 3 4 m s N = 1 2 m s W

55 8 55. Determine the greatest angle u so that the ladder does not slip when it supports the 75-kg man in the position shown. The surface is rather slippery, where the coefficient of static friction at and is ms = 0.3. C 0.25 m G 2.5 m u 2.5 m Free-ody Diagram: The slipping could occur at either end or of the ladder. We will assume that slipping occurs at end. Thus, F = msn = 0.3N. Equations of Equilibrium: Referring to the free-body diagram shown in Fig. b, we have + Fx = 0; : FC sin u>2-0.3n = 0 FC sin u>2 = 0.3N + c Fy = 0; (1) N - FC cos u>2 = 0 T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) FC cos u>2 = N(2) Dividing Eq. (1) by Eq. (2) yields tan u>2 = 0.3 u = = 33.4 Using this result and referring to the free-body diagram of member C shown in Fig. a, we have a + M = 0; FC sin (2.5) - 75(9.81)(0.25) = 0 + Fx = 0; : F sin + c Fy = 0; N cos = (9.81) = 0 2 FC = N F = N N = N Since F 6 (F) max = msn = 0.3(607.73) = N, end will not slip. Thus, the above assumption is correct.

56 *8 56. The uniform 6-kg slender rod rests on the top center of the 3-kg block. If the coefficients of static friction at the points of contact are m = 0.4, m = 0.6, and m C = 0.3, determine the largest couple moment M which can be applied to the rod without causing motion of the rod. C 800 mm M Equations of Equilibrium: From FD (a), : + F x = 0; F - N C = 0 (1) 300 mm 600 mm + c F y = 0; a+ M = 0; N + F C = 0 F C N C M = 0 (2) (3) 100 mm 100 mm From FD (b), + c F y = 0; : + F x = 0; a+ M O = 0; N - N = 0 F - F = 0 F N 1x x2 = 0 Friction: ssume slipping occurs at point C and the block tips, then F C = ms C N C = 0.3NC and x = 0.1 m. Substituting these values into Eqs. (1), (2), (3), (4), (5), and (6) and solving, we have M = N # m = 8.56 N # m N = N N = N F = F = N C = N (4) (5) (6) Since 1F 2 max = m s N = = N 7 F, the block does not slip. lso, 1F 2 max = m s N = = N 7 F, then slipping does not occur at point. Therefore, the above assumption is correct.

57 8 57. The disk has a weight W and lies on a plane which has a coefficient of static friction m. Determine the maximum height h to which the plane can be lifted without causing the disk to slip. z h a y 2a x Unit Vector: The unit vector perpendicular to the inclined plane can be determined using cross product. Then Thus cos g = = (0-0)i + (0 - a)j + (h - 0)k = -aj + hk = (2a - 0)i + (0 - a)j + (0-0)k = 2ai - aj i j k N = * = 3 0 -a h3 = ahi + 2ahj + 2a 2 k 2a -a 0 n = N N = ahi + 2ahj + 2a2 k a25h 2 + 4a 2 Equations of Equilibrium and Friction: When the disk is on the verge of sliding down the plane, F = mn. F n = 0; N - W cos g = 0 N = W cos g F t = 0; W sin g - mn = 0 N = W sin g m Divide Eq. (2) by (1) yields 2a hence sin g = 25h 2 + 4a 2 sin g m cos g = 1 25h 25h 2 + 4a 2 (1) (2) ma 15h h + 4a 2a b 5h + 4a = 1 h = 2 25 am

58 8 58. Determine the largest angle u that will cause the wedge to be self-locking regardless of the magnitude of horizontal force P applied to the blocks. The coefficient of static friction between the wedge and the blocks is ms = 0.3. Neglect the weight of the wedge. P u Free-ody Diagram: For the wedge to be self-locking, the frictional force F indicated on the free-body diagram of the wedge shown in Fig. a must act downward and its magnitude must be F msn = 0.3N. Equations of Equilibrium: Referring to Fig. a, we have + c Fy = 0; 2N sin u>2-2f cos u>2 = 0 F = N tan u>2 T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) Using the requirement F 0.3N, we obtain N tan u>2 0.3N u = 33.4 P

59 8 59. If the beam D is loaded as shown, determine the horizontal force P which must be applied to the wedge in order to remove it from under the beam. The coefficients of static friction at the wedge s top and bottom surfaces are m C = 0.25 and m C = 0.35, respectively. If P = 0, is the wedge self-locking? Neglect the weight and size of the wedge and the thickness of the beam. D 3m 4 kn/m 4m C 10 P Equations of Equilibrium and Friction: If the wedge is on the verge of moving to the right, then slipping will have to occur at both contact surfaces. Thus, F = m s N = 0.25N a nd F = m s N = 0.35N. From FD (a), a+ M D = 0; N cos N sin N = kn = 0 From FD (b), + c F y = 0; : + F x = 0; Since a force P 7 0 when P = 0. N sin sin 10 = 0 N = kn P cos cos 10 P = 5.53 kn = 0 is required to pull out the wedge, the wedge will be self-locking

60 *8 60. The wedge has a negligible weight and a coefficient of static friction m s = 0.35 with all contacting surfaces. Determine the largest angle u so that it is self-locking. This requires no slipping for any magnitude of the force P applied to the joint. P u u 2 2 P Friction: When the wedge is on the verge of slipping, then F = mn = 0.35N. From the force diagram (P is the locking force.), tan u 2 = 0.35N N = 0.35 u = 38.6

61 8 61. If the spring is compressed 60 mm and the coefficient of static friction between the tapered stub S and the slider is m S = 0.5, determine the horizontal force P needed to move the slider forward. The stub is free to move without friction within the fixed collar C. The coefficient of static friction between and surface is m = 0.4. Neglect the weights of the slider and stub. k 300 N/m C Stub: S + c F y = 0; N cos N sin (0.06) = 0 N = N P 30 Slider: + c F y = 0; N cos (29.22) sin 30 = 0 N = 18 N : + F x = 0; P - 0.4(18) sin (29.22) cos 30 = 0 P = 34.5 N

62 8 62. If P = 250 N, determine the required minimum compression in the spring so that the wedge will not move to the right. Neglect the weight of and. The coefficient of static friction for all contacting surfaces is ms = Neglect friction at the rollers. k 15 kn/m P 3 Free-ody Diagram: The spring force acting on the cylinder is Fsp = kx = 15(10 )x. Since it is required that the wedge is on the verge to slide to the right, the frictional force must act to the left on the top and bottom surfaces of the wedge and their magnitude can be determined using friction formula. (Ff)1 = mn1 = 0.35N1 10 (Ff)2 = 0.35N2 Equations of Equilibrium: Referring to the FD of the cylinder, Fig. a, N1-15(103)x = 0 N1 = 15(103)x T an his th d wo sa eir is p rk w le co ro is ill o u vi pr de f a rse de ot st ny s d s ec ro p an o te y ar d le d th t o a ly by e s in f th se for Un te is ss th ite gr w in e ity o g us d S of rk ( stu e o tat th inc de f i es e lu nt ns co w d le tr p or in a uc y k g rn to rig an on in rs h d th g. in t la is e D t w no W iss ea s t p or em ch in er ld m W ina g itt id tio ed e n. We or b) + c Fy = 0; Thus, (Ff)1 = (103)x4 = 5.25(103)x Referring to the FD of the wedge shown in Fig. b, + c Fy = 0; N2 cos N2 sin 10-15(103)x = 0 N2 = (103)x + Fx = 0; : (103)x (103)x4cos (103)x4sin 10 = 0 x = m = 18.3 mm

63 8 63. Determine the minimum applied force P required to move wedge to the right.the spring is compressed a distance of 175 mm. Neglect the weight of and. The coefficient of static friction for all contacting surfaces is m s = Neglect friction at the rollers. k =15kN/m Equations of Equilibrium and Friction: Using the spring formula, F sp = kx = = kn. If the wedge is on the verge of moving to the right, then slipping will have to occur at both contact surfaces. Thus, F = msn = 0.35N and F = msn = 0.35N. From FD (a), P 10 + c F y = 0; From FD (b), N = 0 N = kn + c F y = 0; : + F x = 0; N cos N sin = 0 N = kn P cos sin 10 = 0 P = 2.39 kn